HTMLCanvasElement - Web APIs 编辑

The HTMLCanvasElement interface provides properties and methods for manipulating the layout and presentation of <canvas> elements. The HTMLCanvasElement interface also inherits the properties and methods of the HTMLElement interface.

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Properties

Inherits properties from its parent, HTMLElement.

HTMLCanvasElement.height

The height HTML attribute of the <canvas> element is a positive integer reflecting the number of logical pixels (or RGBA values) going down one column of the canvas. When the attribute is not specified, or if it is set to an invalid value, like a negative, the default value of 150 is used. If no [separate] CSS height is assigned to the <canvas>, then this value will also be used as the height of the canvas in the length-unit CSS Pixel.

HTMLCanvasElement.width

The width HTML attribute of the <canvas> element is a positive integer reflecting the number of logical pixels (or RGBA values) going across one row of the canvas. When the attribute is not specified, or if it is set to an invalid value, like a negative, the default value of 300 is used. If no [separate] CSS width is assigned to the <canvas>, then this value will also be used as the width of the canvas in the length-unit CSS Pixel.

HTMLCanvasElement.mozOpaque

Is a Boolean reflecting the moz-opaque HTML attribute of the <canvas> element. It lets the canvas know whether or not translucency will be a factor. If the canvas knows there's no translucency, painting performance can be optimized. This is only supported in Mozilla-based browsers; use the standardized canvas.getContext('2d', { alpha: false }) instead.

HTMLCanvasElement.mozPrintCallback

Is a function that is Initially null. Web content can set this to a JavaScript function that will be called when the canvas is to be redrawn while the page is being printed. When called, the callback is passed a "printState" object that implements the MozCanvasPrintState interface. The callback can get the context to draw to from the printState object and must then call done() on it when finished. The purpose of mozPrintCallback is to obtain a higher resolution rendering of the canvas at the resolution of the printer being used. See this blog post.

Methods

Inherits methods from its parent, HTMLElement.

HTMLCanvasElement.captureStream()

Returns a CanvasCaptureMediaStream that is a real-time video capture of the surface of the canvas.

HTMLCanvasElement.getContext()

Returns a drawing context on the canvas, or null if the context ID is not supported. A drawing context lets you draw on the canvas. Calling getContext with "2d" returns a CanvasRenderingContext2D object, whereas calling it with "webgl" (or "experimental-webgl") returns a WebGLRenderingContext object. This context is only available on browsers that implement WebGL.

HTMLCanvasElement.toDataURL()

Returns a data-URL containing a representation of the image in the format specified by the type parameter (defaults to png). The returned image is in a resolution of 96dpi.

HTMLCanvasElement.toBlob()

Creates a Blob object representing the image contained in the canvas; this file may be cached on the disk or stored in memory at the discretion of the user agent.

HTMLCanvasElement.transferControlToOffscreen()

Transfers control to an OffscreenCanvas object, either on the main thread or on a worker.

Obsolete methods

HTMLCanvasElement.mozGetAsFile()

Returns a File object representing the image contained in the canvas; this file is a memory-based file, with the specified name. If type is not specified, the image type is image/png.

Events

Listen to these events using addEventListener().

webglcontextcreationerror

Fired if the user agent is unable to create a WebGLRenderingContext or WebGL2RenderingContext context.

webglcontextlost

Fired if the user agent detects that the drawing buffer associated with a WebGLRenderingContext or WebGL2RenderingContext object has been lost.

webglcontextrestored

Fired if the user agent restores the drawing buffer for a WebGLRenderingContext or WebGL2RenderingContext object.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• HTML element implementing this interface: <canvas>